The invention relates generally to an actuator. In particular, the invention relates to an electromechanical actuator for actuating a valve or damper in a fluid distributing system, such as an air conditioning system or a heating system where the actuator must return the valve or damper to a "home" position when power is lost (return-to-normal). Conventional prior art return-to-normal actuators contain a return spring that is almost always coupled to the gear train that transmits power from a drive motor to the output which effects actuation. Because the spring in such a device is always in the gear train, a large drive motor and robust gears are required to supply appropriate torque to drive the load experienced because of the valve or damper being actuated, and also to supply torque to oppose the torque of the spring. As motor size increases, motor current draw also increases. Generally, motor cost also increases with motor size. Such increased current causes thermal problems for the actuator, for the drive electronics, and for the motor. Further, in such prior art devices where the spring is always coupled to the gear train, the spring is cycled every time the drive motor is cycled, thereby presenting significant fatigue problems for the spring. Typically, the life of such prior art return-to-normal actuators is limited by the fatigue life of the spring.
It would, therefore, be useful to have a return-to-normal actuator for use with valves, dampers, or other actuation environments which initially winds a spring or other bias device and then decouples that spring from the gear train. Such a device would allow the motor to drive only the load provided by the damper, valve, or other device being actuated during normal operation. Once power to the actuator is removed, the spring is recoupled to the gear train and the spring drives the output shaft to its predetermined normal position.